Abstract:
A method is described for determining the partition coefficient, K, for radon gas
distributed between an aqueous phase and an organic solvent. The method uses
sequential extractions of radon into equal volume aliquots of organic solvent. The
radon-laden organic liquid is then counted on a liquid scintillation analyzer with alpha-beta
separation. The high quench resistance and counting efficiency of alpha particles
by liquid scintillation methods are ideal for counting a variety of aromatic, aliphatic,
and cyclic organic solvent and scintillation cocktail mixtures. Accurate knowledge of
the instrument counting efficiency, quench, and standard solution activity are not
required. Replicate measurements of the aqueous-organic radon partition coefficient on
benzene, toluene, o-xylene, n-hexane, and cyclohexane showed excellent agreement
with theoretical radon partition coefficients derived from Ostwald solubility
coefficients. The method was also used to determine the radon partition coefficient for
several commercial liquid scintillation solutions. Though performed on lighter-than-water
solvents, the method is potentially amendable to solvents more dense than water.
Knowledge of radon partitioning plays a significant role in the standard liquid
scintillation counting method for analyzing radon in aqueous samples. Radon is unique
in that it distributes itself between the aqueous, cocktail and gas phases within an LSC
vial. With too little cocktail, the vial headspace becomes a significant radon sink. More
cocktail minimizes radon loss to the vial headspace, but with the potential for higher
instrument background. The effect of cocktail volume on method performance when
analyzing radon in 5.0 mL aqueous samples was investigated by preparing samples
having from 5.0 to 15.0 mL of scintillation cocktail floating atop of 5.0 mL of standard
solution. As predicted, radon partitioning played a direct role in the measured counting
efficiency. In all cases, analyzing 5.0 mL of aqueous sample using 5.0 mL of
scintillation cocktail proved to be the optimum analysis protocol.
